This proposal focuses on DNA repair enzymes and signaling proteins with multiple roles in cellular responses to DNA damage. We view these multitasking proteins as potential control nodes that may integrate DNA damage-specific signals and marshal the appropriate repair process(es) to the sites of DNA damage. A frontier area in the field of DNA repair is to understand how the biochemical pathways of DNA repair are coordinated with one another, in a manner analogous to other intracellular signaling pathways. Our immediate efforts are focused on 1) defining the mechanism of substrate selection by mammalian DNA ligase III in DNA damage responses, 2) creating a "chemical genetic switch" to shut off the NER pathway in order to explore other diverse functions of the repair endonuclease ERCC1-XPF, and 3) the enzymatic regulation of Sir2, a protein deacetylase with diverse activities, including DNA repair functions. We are using a structure-based approach to develop small molecule modulators of protein- protein interactions and enzymatic activities that can ultimately be used to probe the physical interactions and functional crosstalk between DNA repair pathways in living cells. Inhibitors of DNA repair activities may ultimately be useful in treating cancers that have become resistant to DNA crosslinkers, alkylating agents, and other DNA-targeted therapies. PUBLIC HEALTH RELEVANCE: We are studying how damaged DNA is repaired. These repair processes are essential for the normal maintenance of our genetic blueprint, but they can also cause resistance to anti-cancer drugs that kill tumors by inflicting damage to DNA. A better understanding of basic DNA repair mechanisms could lead to the development of drugs used to temporarily switch off repair during cancer chemotherapy.